Optical switching device comprising two waveguides whereof both smallest dimensions are less than the guided wavelengths

ABSTRACT

Each waveguide  1, 2  has an elongate shape, comprises an entrance at one end and an exit at the other end and is placed on a surface so that all the entrance ends of all the waveguides can be simultaneously illuminated in one region  3  by said radiation to be switched; according to the invention, each waveguide is produced in a different material; preferably, the waveguides  1, 2  have approximately the same dimensions, thus making it possible to obtain particularly inexpensive switching means.

[0001] The invention relates to the use of radiation waveguides whosetwo smallest dimensions are smaller than the wavelengths of thisradiation, which waveguides are capable, however, of guiding thisradiation thanks to “plasmon” resonance phenomena.

[0002] The article entitled “Plasmon polaritons of metallic nanowiresfor controlling submicron propagation of light”, published on Sep. 15,1999 in the journal Physical Review B, Vol. 60, No. 12, p. 9061-8,written by Jean-Claude Weeber et al., describes the principles ofselective electromagnetic energy transport using metal “nanowaveguides”deposited on a dielectric substrate.

[0003] These elongate metal particles are, for example, deposited on thedielectric substrate using cathode-type lithographic methods.

[0004] This article gives examples illustrating these principles, basedon the use, as “nanowaveguides”, of elongate parallelepipeds whose twosmallest dimensions, namely the height h and the width e, are smallerthan the wavelength of the radiation to be selectively propagated withinthem:

[0005] by illuminating the end of a gold “nanowaveguide” havingdimensions of 1500 nm×30 nm×15 nm, the normalized light intensitydetected at the other end is a maximum (40%) for a wavelength of 835 nmand a minimum (zero) for a wavelength of 633 nm;

[0006] by illuminating the end of a “nanowaveguide”, again made of gold,having dimensions of 1000 nm×30 nm×20 nm, the normalized light intensitydetected at the other end is a maximum for a different wavelength,namely 770 nm.

[0007] Since the maximum transmission of these “nanowaveguides” dependson the wavelength to be transmitted, we may speak of selectiveelectromagnetic energy transport or of a waveguide with spectralselectivity; the means used here to obtain this selectivity is based onthe difference in dimensions, especially the two smallest dimensions, ofthe nanowaveguides which, in this case, are made in the same material.

[0008] That document describes the following example, shown in FIG. 9:

[0009] two identical “nanowaveguides” 1 μm in length are produced usingthe same material, are deposited on a dielectric surface and are placedparallel to each other at a distance of about 0.2 μm apart, so as to beable to simultaneously excite two neighbouring ends of thesenanowaveguides using the same beam; these excitation ends are called“entrance” ends;

[0010] deposited at the other end of each “nanowaveguide” is a gold“nanoparticle”, one having dimensions of 20 nm×30 nm×100 nm and theother dimensions of 30 nm×30 nm×100 nm; and

[0011] the entrance ends of the nanowaveguides are illuminatedsimultaneously and it is found that, depending on the illuminationwavelength, one or other of the nanoparticles scatters radiation.

[0012] Thus, this document teaches that a system comprising twonanowaveguides placed around the same point of illumination or entranceregion and terminating in nanoparticles of different sizes makes itpossible to excite a nearby particle of one or other of thenanowaveguides at the wavelength of the radiation used.

[0013] The means used here to excite the particle is based on thedifference in particle size.

[0014] However, the construction of such a system remains difficult,especially because it requires the deposition of nanoparticles at theend of the nanowaveguides and the excitation of just a single particle,and also because it requires elements of different sizes to be depositedon the dielectric substrate.

[0015] The object of the invention is to avoid this drawback.

[0016] For this purpose, the subject of the invention is an opticalswitching device for switching radiation, comprising a surface providedwith radiation waveguides each having an elongate shape, each comprisingan entrance at one end and an exit at the other end, the two smallestdimensions of each waveguide being smaller than the wavelengths of thesaid radiation, these waveguides being placed on the surface so that allthe entrance ends can be simultaneously illuminated by the saidradiation to be switched, characterized in that each waveguide differsby the material of which it is made.

[0017] The invention may also have one or more of the followingfeatures:

[0018] the said material is chosen from the group comprising gold,silver, aluminium, copper and mixed indium tin oxide;

[0019] the nature of the material of the waveguides and the dimensionsof the waveguides are adapted in order to obtain, within the saidwaveguides, electron plasma resonance for at least one possiblewavelength of the said radiation to be switched;

[0020] The excitation wavelength of the resonance corresponds in generalto quite a broad range of wavelengths centred on a peak at which theresonance is at maximum; preferably, the resonance wavelengths of thesaid waveguides are all between 350 nm and 1 100 nm;

[0021] the waveguides deposited on the same surface have approximatelythe same dimensions.

[0022] The dimensions are therefore precluded from acting as selectivitymeans or as switching means, as in the prior art; because of thisadditional feature, waveguides of different types but of the same sizemay be deposited more economically, using the same process with the samesettings; thus, optical switching of the radiation is then achievedusing waveguides of identical size but different in nature, and thisallows an optical switching device to be fabricated inexpensively.

[0023] The expression “approximately identical dimensions” is understoodto mean the waveguide dimensions that can be achieved using the samedeposition process and the same adjustments; preferably, the said twosmallest dimensions of the waveguides do not exceed 100 nm; typically,the two smallest dimensions of the waveguides are around 40 nm.

[0024] The subject of the invention is also an optical system comprisingone entrance and several exits, characterized in that it comprises anoptical switching device according to the invention, the entrance of thesystem being designed to illuminate simultaneously all the entrance endsof the said waveguides, each of the exit ends of the said waveguidesbeing connected to an exit of the system.

[0025] The invention applies most particularly to a device for opticallyreading, at at least two different wavelengths, digital data stored on amedium, such as an optical disc, and comprising, for reading at thedifferent wavelengths, a device for optically switching the said variouswavelengths.

[0026] Some optical discs for storing digital data have variousrecording layers that can be read simultaneously provided that readlaser beams of different wavelengths are employed, each tailored to onerecording layer.

[0027] On other optical discs for storing digital data, the data isaccessible by luminescence and also requires an optical read deviceoperating at different wavelengths.

[0028] Other optical discs, such as for example those in which the datais stored in the form of “plasmons”, require read devices operating atat least two different wavelengths.

[0029] The optical read devices for reading such data media thereforeinclude means for switching the various read or luminescencewavelengths; for this purpose, it is advantageous to use an opticalswitching device for switching the data read radiation, comprising asurface provided with radiation waveguides, each having an elongateshape, each comprising an entrance at one end and an exit at the otherend, the two smallest dimensions of each waveguide being smaller thanthe wavelengths of the said radiation, these waveguides being placed onthe surface so that all the entrance ends can be simultaneouslyilluminated in one region by the said read radiation.

[0030] Preferably, each waveguide differs by the material of which it ismade.

[0031] Such a switching device is very light and very compact; it cantherefore be very easily integrated into the read head of the readdevice.

[0032] The invention relating to the switching device will be moreclearly understood on reading the description that follows, given by wayof non-limiting example and with reference to the appended figures inwhich:

[0033]FIG. 1 shows a diagram of the device according to theinvention—the upper part in top view and the lower part in side view;and

[0034]FIG. 2 shows the normalized scattered intensity at the exit end ofthe waveguides of the device according to the invention as a function ofthe wavelength.

[0035] Using conventional cathode-based lithography methods, waveguides1, 2 of identical dimensions, but differing in type, are deposited on aglass substrate 4 having an index of 1.5; the device shown schematicallyin FIG. 151 is thus obtained.

[0036] The first waveguide 1 is made of aluminium and the secondwaveguide 2 is made of gold; the common dimensions of the waveguides are2 000 nm×40 nm×40 nm; the waveguides are placed so as to be parallel andat a distance of about 0.1 μm apart, that is to say at a distance largeenough to avoid any direct coupling effect between them but small enoughfor it to be possible for the two neighbouring ends of thesenanowaveguides to be simultaneously excited using the same light beam,these two ends being consequently termed entrance ends and the otherends termed exit ends.

[0037] We will now show how the device thus obtained can be used as anoptical switching means.

[0038] As shown in FIG. 1, the beam of the radiation to be switched ismade to converge on an entrance region 3 straddling the entrance ends ofthe two guides.

[0039] This incident radiation may be focused through the glasssubstrate by a suitable optical device such as, for example, animmersion objective; it is possible to choose a value close to 1.5 asrefractive index of the immersion oil of this objective; it is possibleto choose a value close to 0.9 as the optical aperture of thisobjective.

[0040] Using suitable computing means known per se, the scatteredintensity at the exit end of the guides under the abovementionedexcitation conditions is evaluated; the results were obtained at adistance of around 30 nm above the upper surface of the exit end of theguides; the results are normalized to the incident light and are plottedin FIG. 2—the continuous line is for the aluminium waveguide and thebroken line is for the gold waveguide; the peaks in these figurescorrespond to the wavelengths capable of exciting the electron plasmaresonance within the corresponding waveguide; for example, a maximumresonance is observed at 489 nm within the aluminium waveguide, and isobserved at 790 nm within the gold waveguide.

[0041] Under these excitation conditions, by observing the spatialdistribution of the electromagnetic field intensity along thewaveguides:

[0042] under 489 nm excitation, it is found that the electromagneticwave propagates only in the aluminium waveguide;

[0043] under 694 nm excitation, it is found that the electromagneticwave propagates simultaneously in both waveguides; and

[0044] under 790 nm excitation, it is found that the electromagneticwave propagates essentially only in the gold waveguide.

[0045] Thus, the system thus obtained can therefore be really used as anoptical switching means.

[0046] According to an alternative embodiment of the invention, severalnanowaveguides, of identical dimensions but of different materials, aredeposited on the glass substrate so as to obtain more than two exits,the entrance being still shared between the various nanowaveguides;according to this embodiment, the nanowaveguides are arranged, forexample, as a star around the entrance or excitation region on which theincident beam converges; these various materials will be chosen in amanner known per se so that the wavelengths for maximum resonance do notoverlap.

[0047] The optical switching device according to the invention is lessexpensive than those of the prior art, especially because all thenanowaveguides of this device have the same size and can be producedusing the same deposition process with the same settings.

[0048] The optical switching device according to the invention mayadvantageously be integrated into an optical read device for reading adigital data storage medium.

1. Optical switching device for switching radiation, comprising asurface provided with radiation waveguides (1, 2) each having anelongate shape, each comprising an entrance at one end and an exit atthe other end, the two smallest dimensions of each waveguide beingsmaller than the wavelengths of the said radiation, these waveguidesbeing placed on the surface so that all the entrance ends can besimultaneously illuminated in a region (3) by the said radiation to beswitched, characterized in that each waveguide differs by the materialof which it is made.
 2. Optical switching device according to claim 1,characterized in that the said material is chosen from the groupcomprising gold, silver, aluminium, copper and mixed indium tin oxide.3. Optical switching device according to any one of the precedingclaims, characterized in that the nature of the material of thewaveguides and the dimensions of the waveguides are adapted in order toobtain, within the said waveguides, electron plasma resonance for atleast one possible wavelength of the said radiation to be switched. 4.Optical switching device according to claim 3, characterized in that theresonance wavelengths of the said waveguides are all between 350 nm and1 100 nm.
 5. Optical switching device according to any one of thepreceding claims, characterized in that the waveguides deposited on thesame surface have approximately the same dimensions.
 6. Opticalswitching device according to any one of the preceding claims,characterized in that the said two smallest dimensions of the waveguidesdo not exceed 100 nm.
 7. Optical system comprising one entrance andseveral exits, characterized in that it comprises an optical switchingdevice according to any one of the preceding claims, the entrance of thesystem being designed to illuminate simultaneously all the entrance endsof the said waveguides, each of the exit ends of the said waveguidesbeing connected to an exit of the system.
 8. Device for opticallyreading, at at least two different wavelengths, digital data stored on amedium, such as an optical disc, and comprising, for reading at thedifferent wavelengths, an optical switching device for switching thedata read radiation, comprising a surface provided with radiationwaveguides, each having an elongate shape, each comprising an entranceat one end and an exit at the other end, the two smallest dimensions ofeach waveguide being smaller than the wavelengths of the said radiation,these waveguides being placed on the surface so that all the entranceends can be simultaneously illuminated in one region by the said readradiation.
 9. Device according to claim 8, characterized in that eachwaveguide differs by the material of which it is made.